The incident happened when a group of 70 Serbs were traveling to Gazimestan to celebrate St. Vitus Day, Tanjug reported.
They were stopped by Kosovo police in Vucitrn, who ordered them to go back through Merdare to central Serbia.
A scuffle ensued and police opened fire on the group.
Ten Serbs were treated for injuries at the scene, while another 10 were taken to a local hospital for treatment of more serious injuries.
"One person with life-threatening injuries was taken by ambulance from Prokuplje to Nis," Kursumlija Health Center Director Ljiljana Arsenijevic told Tanjug.
Kosovo's Interior Minister Bajram Rexhepi told Radio Free Europe 32 police officers were injured in an incident earlier Thursday.
"The policemen were injured when a group of Serbs, who were trying to cross over into Kosovo, threw rocks at them," he said.
A total of 50 people were injured during the incidents.
The United Nations condemned the actions of the police in a release.
"The United Nations envoy for Kosovo has condemned the incidents that occurred there today, particularly the attack on buses carrying a number of children of Serbian nationality, and called on all parties to refrain from actions that could worsen the situation," the release said.
Farid Zarif, the secretary-general's special representative, called on "the authorities to conduct a complete and thorough investigation into the incidents," the statement said.